Quality control for decorative anodized coatings

Above and beyond their protective function, decorative anodized coatings must also meet certain design requirements: The exact coloring plays an important role. But even small differences in the aluminum alloy can significantly affect the final hue. Therefore, an accurate review of the raw materials during incoming goods inspection is necessary to achieve consistent coloration and to prevent production wastage.

Not every aluminum alloy can be anodized for decorative purposes; therefore, mainly AL99, AIMg or AlMgSi alloys are used. For best results, these aluminum alloys must also be of highly pure anodizing quality. But even with supposedly the same alloys, slight differences in composition may occur from manufacturer to manufacturer and from batch to batch, which can then lead to significant color deviations during the anodizing process.

For decorative anodized finishes, the exact hue is actually an important aspect of the quality; discrepancies in coloration can drive manufacturing costs up, because they can require extensive reworking to correct problems or even a new production run. In order to avoid such issues, it is necessary to know whether one is indeed working with the exact same aluminum alloy – or not. The best way to check this is to determine the electrical conductivity of the raw material or semi-finished products before the anodizing process, as this parameter is sensitive to even small variances in composition.

Fig.1: Different color options for anodized cartridges

As an example, four sheets each of the alloys AIMg3 and AlMgSi0.5 were color-anodized all together, resulting in two different shades of blue. The electrical conductivity of the two base materials showed significant differences, as seen in Table 1.

Color variant A

part 1A

part 2A

part 3A

part 4A

Mean value [MS/m]

31.33

31.26

31.20

31.27

Standard deviation [MS/m]

0.01

0.02

0.01

0.01

No. of measurements

10

10

10

10

Color variant B

part 1B

part 2B

part 3B

part 4B

Mean value [MS/m]

21.58

21.57

21.57

21.59

Standard deviation [MS/m]

0.01

0.01

0.01

0.01

No. of measurements

10

10

10

10

Tab.1: Conductivity measurements on color-anodized plates from the same anodizing process which resulted in two different shades of blue. The plates that turned out in color variant A were made of AIMg3 whereas those with color variant B were of AlMgSi0.5.

The conductivity of the plates was measured with the SIGMASCOPE® SMP10 and the ES40 probe, both from FISCHER. Using the non-destructive phase-sensitive eddy current method, this instrument quickly and accurately determines the electrical conductivity of non-ferrous metals. The ES40 probe can handle a wide range of measuring frequencies (60-480 kHz), making it suitable for a variety of material thicknesses. Since the electrical conductivity is strongly temperature dependent, it also has an integrated temperature sensor. Furthermore, in order to allow for calibration on the supplied flat calibration standards when measuring rounded parts, curvature corrections starting at Ø 6 mm ensure consistently high precision: the diameter of the sample’s curvature is simply entered into the instrument.

To avoid differences in the final color of decorative anodized coatings, checking the conductivity of the raw material before anodizing is an appropriate quality control step. The combination of SIGMASCOPE® SMP10 and ES40 probe is ideal for measuring the electrical conductivity of non-ferrous metals like aluminum alloys. For more information please contact your local FISCHER representative.